This invention relates generally to electric incandescent lamps, and more particularly to tubular lamps having an axially extending coiled filament. Still more particularly, the invention relates to filament supports for tungsten-halogen lamps having tubular envelopes of the double ended type with coiled coil filaments, i.e., filaments formed by winding the filament wire to form a helical primary coil which is again helically coiled to form the secondary coil of the coiled coil filament.
A persistent problem in the manufacture of such lamps using a coiled coil filament is that of providing support for the center of the filament to prevent damage to the filament due to vibration and/or sagging of the center of the hot filament. Accordingly, in tubular lamps of the double ended type having an axially extending filament connected at its ends to lead-in conductors sealed through opposite ends of the envelope, it has been the practice to provide auxillary support members at one or more locations on the filament, intermediate its ends, to support the filament from the envelope walls and maintain it generally at the axis of the envelope. For a filament in the form of a simple helical coil, a widely used form of support comprises a wire member having one end coiled around the exterior of several turns of the helical filament coil and extending therefrom, usually in the general form of a spiral, to be circumferentially engageable with the inner wall of the envelope. However, such a support is not suitable for coiled coil filaments, particularly filaments of high wattage and therefore relatively large size. Previous attempts to provide a suitable support for coiled coil filament lamps have suffered from various disadvantages such as failure to firmly support the filament in the axis of the envelope, difficulty in making the support and assembling it with the other lamp components, and electrical short circuiting of portions of the filaments in varying degrees or requiring additions to the length of the filament thereby nullifying, to some degree, the desireable compactness of a coiled coil filament.
For example, U.S. Pat. Nos. 3,194,999; 3,195,000 and 3,678,319 illustrate various filament support arrangements in which a portion of the support member is fastened or secured in some manner to the center of the filament, and another end of the support is wedged into the tipped off area of the exhaust tubulation of the envelope. Such support methods, however, require considerably costly techniques to assure that the support is wedged ridgely into the "tip" by molten quartz while not extending through the quartz tip, which could cause the envelope to leak.
U.S. Pat. Nos. 3,270,238; 3,335,312; 3,736,455 and 3,784,865 illustrate filament support arrangements wherein one portion of the support member is fastened to or looped around the center or another intermediate portion of the filament, and the end or ends of the support are wedged into the press seal area of the envelope. Such methods require costly techniques at assembly and/or at press sealing to assure the correct turns per inch for each half of the coiled coil filament. Further such constructions also add considerable cost to the lamp.
U.S. Pat. No. 3,195,000 describes a filament support arrangement in which the coiled coil filament is interrupted or separated at a point intermediate its ends to define separate sections or segments, and the adjacent interrupted ends are joined by a support member comprising a wire portion which is formed to conform to at least a portion of a turn of the secondary coiling of the filament. The support further includes an enlarged helical portion which engages the inner wall of the lamp envelope. Clearly, this approach introduces complexities into both the assembly and structure of the lamp, and reliance upon a single helical wall support for maintaining proper positioning of the filament does not offer the enhanced degree of reliability provided by the structure to be defined hereinafter.
U.S. Pat. No. 3,521,112 describes a support member which loosely encircles the filament and also engages the envelope wall. The primary embodiments illustrate one end of the support member secured in the pinch seal of the lamp. In another embodiment described, however, a helical wire support about the filament, and positioned centrally within the envelope, has legs which extend in both directions close to the pinch seals but are not anchored therein. Although comparatively simple, such a support arrangement appears to permit a greater degree of tolerance in support and/or filament coil movement than is permissable in many applications.
Hence, while the aforementioned prior art supports hold the filament in the desired center location of the envelope and tend to prevent damage to the filament due to vibration, they are comparatively complicated, more costly, or lack the degree of reliability against filament failure that is desired in a number of applications.